Thanks for that, Imre.

I'm a mathematical dunderhead so have to see these things in very simple
terms. I can summarise the two key points of FFT very briefly, and do so in
order to help other mathematically-challenged people.

Firstly, the maximum frequency you can extract is half the sampling
frequency.  In other words, if you sample at 10kHz then the highest
frequency you can pull out is 5kHz (this is known as the Nyquist frequency).

Secondly, the maximum *resolution* is 1 / duration of sound bite.  In other
words, if your sound bite is 100ms long, then the output of the FFT is in
steps of 10Hz.  If it's 2 seconds long, the steps are 0.5Hz apart.  Note
that this is *independent* of the sampling frequency.

There is one last thing to emphasise: filter out everything above the
Nyquist frequency *before* you do the sampling.  In the example 1/ above
with a 10kHz sampling frequency, filter out everything above 5kHz.  The
reason for this is that frequencies above 5kHz get "mirrored" back into the
0 to 5kHz range and give you invalid results.

Hope this is a useful summary.

Steve Thackery
Suffolk, England.
Web Site: http://www.btinternet.com/~stevethack/